Spacecraft that are launched into space are typically designed with one or more devices or systems that allow the spacecraft to unload accumulated angular momentum. As an example, even in space, a spacecraft can be subject to a number of perturbing environmental forces. One such force, such as affecting satellites in a low-earth orbit (LEO), can be a magnetic force resulting from an interaction between currents that are generated within the spacecraft and the magnetism of the poles of the celestial body. Another such force can include solar pressure that is applied to the spacecraft from solar radiation being reflected from the body of the spacecraft. Thus, the spacecraft may be configured to accumulate and unload angular momentum in order to counteract the perturbing environmental forces that are applied in space.
In addition, a typical spacecraft may need to generate electrical energy to power onboard avionics. As an example, a spacecraft may include solar array panels that that convert solar radiation into electrical energy. Accordingly, the spacecraft may implement a device that orients the solar array towards the sun in a manner so as to optimize the amount of solar radiation that is incident on the solar array panels throughout the orbit.
FIG. 1 demonstrates an example of a typical spacecraft system 10. The system 10 demonstrates a spacecraft 12, demonstrated in the example of FIG. 1 as a satellite, in an orbital pattern 14 around Earth 16. The orbital pattern could be a geosynchronous earth orbit (GEO), such that the spacecraft 12 is orbiting Earth 16 at an approximately equatorial orbit, as demonstrated in the example of FIG. 1. The spacecraft 12 is demonstrated in four separate locations in the orbital pattern 14. At each of the four locations, the spacecraft 12 is oriented such that solar array panels 18 on the spacecraft 12 are positioned to optimize the receipt of solar radiation, demonstrated as dashed lines 20, from the Sun 22. The solar radiation can thus be converted into electrical power to drive a load and/or to charge a battery, such that the spacecraft 12 can still drive the load when the solar radiation is unavailable, such as in a shadow region 24 behind Earth 16 relative to the Sun 22.
In order to maintain the orientation of the spacecraft 12 to optimize the receipt of solar radiation and in order to counteract perturbing environmental forces, such as solar pressure from the solar radiation 20, the spacecraft 12 can include one or more systems for accumulating and unloading momentum to maintain a direction of pointing of the spacecraft. As an example, the spacecraft 12 can include one or more reaction wheels that can be commanded to spin to generate a torque, or angular momentum vector relative to a center of mass to rotate the spacecraft 12, such as to counteract the environmental torque on the spacecraft 12 and maintain a directional vector of the spacecraft 12. However, the momentum that can be accumulated by the reaction wheels is finite, such that the reaction wheels can saturate, and thus may not be able to accumulate additional momentum and maintain spacecraft pointing. Therefore, the spacecraft 12 can also include thrusters disposed along the body of the spacecraft that can be ignited to unload momentum from the reaction wheels. However, the additional momentum unloading thrusters, along with the associated fuel, occupy significant space and weight on the spacecraft 12. Furthermore, the amount of fuel for the momentum unloading thrusters is also finite, which could effectively shorten the mission life of the spacecraft 12.